A well known use of storage phosphors is in the production of X-ray images. In U.S. Pat. No. 3,859,527 a method for producing X-ray images with a photostimulable phosphor, which are incorporated in a panel is disclosed. The panel is exposed to incident pattern-wise modulated X-ray beam and as a result thereof the phosphor temporarily stores energy contained in the X-ray radiation pattern. At some interval after the exposure, a beam of visible or infra-red light scans the panel to stimulate the release of stored energy as light that is detected and converted to sequential electrical signals which (are) be processed to produce a visible image. For this purpose, the phosphor should store as much as possible of the incident X-ray energy and emit as little as possible of the stored energy until stimulated by the scanning beam. This is called “digital radiography” or “computed radiography”.
Since in the above described X-ray recording systems the X-ray conversion screens are used repeatedly, it is important to provide them with an adequate topcoat for protecting the phosphor containing layer from mechanical and chemical damage. This is particularly important for photostimulable radiographic screens where screens are often transported in a scanning module—wherein the stimulation of the stored energy takes place—while not being encased in a cassette but is used and handled as such without protective encasing.
The image quality that is produced by any radiographic system using phosphor screen thus also in a digital radiographic system, depends largely on the construction of the phosphor screen. Generally, the thinner a phosphor screen at a given amount of absorption of X-rays, the better the image quality will be. This means that the lower the ratio of binder to phosphor of a phosphor screen, the better the image quality, attainable with that screen, will be. Optimum sharpness can thus be obtained when screens without any binder are used. Such screens can be produced, e.g., by physical vapor deposition, which may be thermal vapor deposition, sputtering, electron beam deposition or other of phosphor material on a substrate. However, this production method can not be used to produce high quality screens with every arbitrary phosphor available. The mentioned production method leads to the best results when phosphor crystals with high crystal symmetry and simple chemical composition are used.
The use of alkali metal halide phosphors in storage screens or panels is well known in the art of storage phosphor radiology and the high crystal symmetry of these phosphors makes it possible to provide structured,as well as binderless screens.
It has been disclosed that when binderless screens with an alkali halide phosphor are produced it is beneficial to have the phosphor crystal deposited as some kind of piles, needles, tiles, etc. to increase the image quality than can be obtained when using such a screen. In, e.g., U.S. Pat. No. 4,769,549 it is disclosed that the image quality of a binderless phosphor screen can be improved when the phosphor layer has a block structure shaped in fine pillars. In e.g. U.S. Pat. No. 5,055,681 a storage phosphor screen comprising an alkali halide phosphor in a pile-like structure is disclosed. Also in EP-A-1 113 458 a phosphor panel with a vapor deposited CsBr:Eu phosphor layer wherein the phosphor is present as fine needles separated by voids is disclosed for optimising the image quality.
Unfortunately such needle shaped phosphors are quite brittle and the phosphor panels are prone to physical damage after only a few cycles in the scanning apparatus. It has been proposed to strengthen the screens or panels by applying a protective layer on top of the vapor deposited phosphor layer. Such a protective overcoat is described in published EP-A-392 474. Also the use of radiation curable coating to form a protective top layer in a X-ray conversion screen is described e.g. in EP-A-209 358 and JP-A-86 176 900 and U.S. Pat. No. 4,893,021. For example, the protective layer comprises a UV cured resin composition formed by monomers and/or prepolymers that are polymerized by free-radical polymerisation with the aid of a photoinitiator. The monomeric products are preferably solvents for the prepolymers used.
In co-pending EP-Application No. 01000694, filed Dec. 3, 2001, a binderless stimulable phosphor screen is disclosed having a support and a vapor deposited phosphor layer and a protective layer on top of said phosphor layer characterized in that said vapor deposited phosphor is needle-shaped and said phosphor needles have a length, L and voids between them and wherein said protective layer fills said void for at most 0.10 times L. By doing so the strength of the panel is increased. In U.S. Pat. No. 4,947,046 it is disclosed that the voids between needle phosphor can be filled with colorants, dyes and/or pigments, thus enhancing the image quality.
Although all screens disclosed in this prior art can yield X-ray images with good quality, there is still a need for storage phosphor screens with increased physical strength that can withstand the wear and the tear of transporting.